Vinylidene fluoride resins are excellent in weatherability and chemical resistance compared with general-purpose resins, such as polyolefins, and also have heat resistance, strength, etc., so that there have been made many proposals regarding porous membranes of vinylidene fluoride resins used as microfiltration membranes for treatment of water (or liquid), particularly hollow fiber-form porous membranes, i.e., porous hollow fibers, and processes for production thereof (e.g., Patent documents 1-4 listed below). In the case where porous hollow fibers are actually used as a filter element for treatment of water (or liquid, hereinafter representatively called “water”), porous hollow fibers (water-collecting tubes) cut in a uniform length of generally ca. 0.2-2 m are bundled to form a module, and in many cases, water to be treated is supplied from the outer surface of the thus-bundled porous hollow fibers and is filtrated through pores formed in the walls into the hollow parts thereof to form treated water, which is caused to flow out of both ends of the porous hollow fibers to obtain filtrated water. A porous hollow fiber used for treatment of water by microfiltration is required of many properties, which are, for example, (i) a uniform pore diameter distribution of a size suitable for removing particles to be removed, (ii) large mechanical durabilities (tensile strength and pressure resistance), (iii) a large treatment capacity (water permeation rate) per single fiber, (iv) a large capacity per volume when formed into a module, and (v) chemical resistance. However, conventional porous hollow fibers of vinylidene fluoride resins have not satisfied all of these requirements for microfiltration.
Patent document 1: JP-A 63-296939
Patent document 2: JP-A 63-296940
Patent document 3: JP-A 3-215535
Patent document 4: JP-A 11-319522